#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#include <time.h>
#include <stack>
#include <array>
#include <list>
#include <atcoder/all>
#define popcount __builtin_popcount
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef pair<int, int> P;
struct SCC{
    vector<vector<int>> g, gr;
    int n, k;
    vector<int> cmp, vs;
    vector<bool> used;
    void dfs(int x){
        used[x]=1;
        for(auto y:g[x]){
            if(!used[y]) dfs(y);
        }
        vs.push_back(x);
    }
    void rdfs(int v, int k){
        used[v]=1;
        cmp[v]=k;
        for(auto y:gr[v]){
            if(!used[y]) rdfs(y, k);
        }
    }
    SCC(const vector<vector<int>> &g):g(g), n(g.size()), cmp(n), used(n){
        gr.resize(n);
        for(int x=0; x<n; x++) for(auto y:g[x]) gr[y].push_back(x);
        for(int i=0; i<n; i++){
            if(!used[i]) dfs(i);
        }
        fill(used.begin(), used.end(), 0);
        k=0;
        for(int i=vs.size()-1; i>=0; i--){
            if(!used[vs[i]]) rdfs(vs[i], k++);
        }
    }
};
int main()
{
    int n, m; cin>>n>>m;
    vector<P> g[1010], gr[1010];
    vector<vector<int>> g1(n);
    int a[2020], b[2020];
    for(int i=0; i<m; i++){
        cin>>a[i]>>b[i];
        a[i]--; b[i]--;
        g[a[i]].push_back({b[i],i});
        gr[b[i]].push_back({a[i], i});
        g1[a[i]].push_back(b[i]);
    }
    SCC scc(g1);
    if(scc.k>1){
        cout<<-1<<endl;
        return 0;
    }
    if(n==m){
        cout<<-1<<endl;
        return 0;
    }
    int ans[2020]={};
    const int INF=1e9+7;
    int mn=INF;
    for(int i=0; i<n; i++){
        int d[2][1010];
        fill(d[0], d[0]+n, INF);
        fill(d[1], d[1]+n, INF);
        queue<P> que;
        que.push({i, 0});
        d[0][i]=0;
        while(!que.empty()){
            P p=que.front();que.pop();
            int x=p.first, t=p.second;
            for(auto q:g[x]){
                int y=q.first;
                if(d[t^1][y]>d[t][x]+1){
                    d[t^1][y]=d[t][x]+1;
                    que.push({y, t^1});
                }
            }
        }
        if(d[1][i]<mn){
            mn=d[1][i];
            fill(ans, ans+m, 0);
            P p=P(i, 1);
            int hoge=1;
            while(p!=P(i, 0)){
                int x=p.first;
                for(auto q:gr[x]){
                    if(d[p.second^1][q.first]==d[p.second][x]-1){
                        ans[q.second]=hoge;
                        hoge^=1;
                        p=P(q.first, p.second^1);
                        break;
                    }
                }
            }
        }
    }
    if(mn==INF || mn==n) cout<<-1<<endl;
    else{
        for(int i=0; i<m; i++){
            if(ans[i]) cout<<'R';
            else cout<<'G';
        }
        cout<<endl;
    }
    return 0;
}